The term thermophoresis refers to a phenomenon that occurs when a mixture of solid particles and fluid is exposed to a temperature gradient. The “thermophoretic force” that causes this phenomenon arises from interactions between fluid molecules and solid particles due to the temperature gradient. Because a fluid's temperature is a measure of the average velocity of that fluid's molecules, a temperature gradient within a fluid is essentially a gradient in average velocity and associated kinetic energy.
Higher temperature, faster moving fluid molecules collide more frequently, and at greater energy, with a particle than do lower temperature molecules. Thus, the temperature gradient produces an asymmetric interaction that tends to impart kinetic energy to the particle in the direction of lower temperature.
Past efforts have shown that thermophoresis plays a role in attracting particles from a hot gas mixture to a relatively cold surface. For example, in the case of a hot particle-gas mixture flowing within a cooler conduit, the conduit surfaces provide thermal boundary conditions that produce a sheath of colder gas surrounding the central core of the mixture. This thermal gradient produces a thermophoretic force that moves particles toward the conduit surfaces.